Optimal value of fractional exhaled nitric oxide in inhaled corticosteroid treatment for patients with chronic cough of unknown cause

Background: Chronic cough related to eosinophilic inflammation has become an important cause of cough in recent years. Although fractional exhaled nitrogen oxide (FeNO) measurement is an important monitoring tool, its lack of reference level may limit its usefulness. This study aimed to determine the optimal FeNO level to guide inhaled corticosteroid (ICS) management in patients with chronic cough. Methods: This retrospective study reviewed the medical records of patients with chronic cough for more than 8 weeks. Based on the selection criteria, data were recalculated to determine the optimal cut-off value of FeNO for initiating ICS treatment in such patients. Results: The medical records of 81 eligible patients were reviewed. Cough improved completely in 38 (Group A) of 44 patients who were treated with ICS but not in the other six patients (Group B). In the 37 patients without ICS treatment, cough improved in 32 (Group C) by specific treatment aimed at the possible causes of the cough, whereas the remaining five (Group D) had persistent cough. The patients were further subdivided into Category 1 (all patients in Group A) and Category 2 (all patients in Groups B and C). Receiver operating characteristic analysis showed that an FeNO of 33.9 ppb was the best cut-off value for using ICS (sensitivity 94.7%, specificity 76.3%) in patients with chronic cough. In fourteen of 15 patients with bronchial hyperresponsiveness (BHR) or borderline BHR, the FeNO values were equal or higher than 33.9 ppb. Obstructive ventilatory impairment was only seen in two patients. Conclusion: Using FeNO measurement as a rapid, noninvasive diagnostic tool for patients with chronic cough, ICS can be prescribed if the FeNO is 33.9 ppb or more.